The German President has begged Poland for forgiveness at the start of memorial ceremonies to mark 80 years since the outbreak of the Second World War.
Speaking in the country's native language at a poignant service in Wielun, Poland, where the first Nazi bombs fell, he told the nation that Germans 'committed a great crime against humanity in Poland' and 'I bow my head before the Polish victims of Germany's tyranny'.
The sombre statement came as world leaders, including US vice president Mike Pence and German chancellor Angela Merkel, along with delegations from up to 40 other countries, to mark the beginning of a war that would cost the lives of tens of millions of people.
At dawn this morning, following the remembrance service in Wielun, London mayor Sadiq Khan joined officials at a ceremony near the Westerplatte memorial in Gdansk, Poland, where Polish and Nazi forces clashed for the first time during the Second World War. World leaders have gathered for an international ceremony in Warsaw and at the Pilsudski Square, named after 20th-century independence army leader Jozef Pilsudski.
Hundreds also massed in Berlin cathedral for a service to mark the beginning of the war.
When German tanks rolled across the border on September 1 1939 it marked the start of a bloody occupation that would cost the lives of six million Poles and countless Jews, which would then be followed by communist rule supported by the Soviets before the iron curtain began to unravel in 1989.
Notable absences at the anniversary observances include Vladimir Putin, who was not invited, Donald Trump, who cancelled his trip at the last minute due to a hurricane, and UK Prime Minister Boris Johnson.
Speaking at the ceremony also attended by the Polish President Andrzej Duda, the German President said: 'I bow my head before the victims of the attack on Wielun. I bow my head before the Polish victims of Germany's tyranny. And I ask forgiveness.
'It is the Germans who committed a crime against humanity in Poland.
'Anyone who claims it is over, that the national-socialists' reign of terror over Europe is a marginal event in German history judges that for himself.'
Polish President Andrzej Duda also gave a speech in Wielun, where he denounced Nazi Germany's attack on Poland, calling it 'an act of barbarity' and 'a war crime'.
However, he also thanked the German President's words, saying: 'I am convinced that this ceremony will go down in the history of Polish-German friendship.'
US vice president Mike Pence has also been seen getting off a plane ahead of a sombre service that he will attend in Warsaw's Pilsudski Square, named after 20th-century independence army leader Jozef Piludski.
German chancellor Angela Merkel has been pictured arriving at the ceremony in Warsaw, which will also be attended by delegations from up to 40 other countries.
London's mayor was also photographed standing shoulder to shoulder with the European commission vice president Frans Timmermans and the mayor of Gdansk Aleksandra Dulkiewicz.
In a statement, Sadiq Khan said he was there to 'bang the drum' for London and 'spread the message that, however Brexit turns out, the city will always be open for business, trade and talent and a welcoming home for Polish citizens'.
'With far-right extremism on the rise on a scale we have not seen for decades, I will also be using this visit to make the positive case for tolerance and inclusion in London, Poland and across Europe.'
Writing in The Observer today, Sadiq Khan has also slammed Boris Johnson and Donald Trump as 'extreme right-wing leaders' and claimed that the lessons of the Second World War are 'in danger of being forgotten'.
He also labelled the US president the 'global poster-boy for white nationalism' and accused him of being an inspiration for right-wing leaders around the world.
UK prime minister Boris Johnson has also paid his respects to the 'bravery' of Poles during the Second World War.
Speaking in a video message filmed in 10 Downing Street, he said: 'On almost every front, Poles fought magnificently for the Allied cause.
'They served alongside the British Army storming the Nazi fortress at Monte Casino, vanquishing Rommel in North Africa and took off from British airports to defend British skies in 1940.'
'Britain has been proud to stand with Poland. Our friendship, born in the struggle against tyranny, is symbolised today by our unbreakable commitment to the NATO alliance.'
The memory of World War Two is a major issue in Poland, with the ruling Law and Justice party (PiS) using it to whip up nationalist feeling by claiming western countries have not recognised the extent of the country's suffering and bravery under Nazi occupation.
Writing in German newspaper Die Welt on Friday, two days before the ceremonies, prime minister Mateusz Morawiecki said: 'The experience of Poland in the Second World War greatly differed from western European countries. The occupation of France and Poland was incomparable.'
Articles in a similar vein are due to appear in newspapers across Europe and the United States this weekend, which are being funded by a foundation financed by state companies.
As part of its campaign, the Polish National Foundation also paid for supplements in some newspapers consisting of a copy of their front pages from Sept. 2, 1939, that highlighted the Nazi German army's attack on Poland.
It is also becoming a central campaign theme ahead of Poland's general election, due on October 13 this year, with the ruling PiS accusing other parties of failing to protect the country's image on the world stage.
In a move likely to increase tensions between Poland and Germany, fuelled under the PiS government, Morawiecki renewed calls for wartime reparations in recent days.
Germany, one of Poland's biggest trade partners and a fellow member of the European Union and NATO, says all financial claims linked to World War Two have been settled.
The cancellation of Trump's visit is a disappointment to the PiS government, which is seen as one of Washington's closest allies in Europe. Polish and U.S. officials have said another visit could be scheduled in the near future.
Opinion polls show PiS is likely to win the October ballot. The party's ambition is to galvanise voters and disprove critics by winning a majority that would allow it to change the constitution.
© Associated Newspapers Ltd.